The goal of this proposal is to use genetically engineered myoblasts as vehicles for gene delivery for the treatment of immune deficiency disorders and cancer. Our approach capitalizes on a unique characteristic of muscle precursor cells. Myoblasts introduced into muscle by injection cross the basal lamina and fuse into pre-existing mature myofibers that are innervated and vascularized. As a result, injected myoblasts are not merely tolerated but nurtured, leading to longterm retention. We have previously demonstrated that myoblasts genetically engineered to express human growth hormone (hGH) stably deliver the hormone to the circulation for a period of three months. We now propose to use myoblast-mediated gene transfer to deliver two recombinant hematopoietic colony stimulating factors, M-CSF and GM-CSF. To date, therapeutic effects have not been reported using cell-mediated gene therapy. Initial studies will focus on gene transfer to correct a mouse model with a genetic defect. The osteopetrotic (op) mouse has a mutation within the M-CSF gene abrogating expression of the M-CSF gene and protein, leading to a severe deficiency in cells of the osteoclast and macrophage lineage. Implantation of genetically engineered myogenic cells into muscle of genetically defective animals will determine whether adequate levels of functional cytokine result from M-CSF gene transfer. Subsequent studies will determine whether myoblast-mediated delivery of CSFs is useful in overcoming dose-limiting toxicity resulting from chemotherapy in a mouse tumor model. Currently the major limitation to using higher doses of chemotherapy to kill cancer cells is the toxicity to bone marrow. The ability to achieve long-term systemic expression of modulators of immune and hematopoietic systems by gene therapy should have a profound impact on the clinical management of acquired diseases such as cancer and AIDS.